This invention relates generally to magnetic recording media, and more particularly to patterned magnetic recording disks with discrete magnetic regions or islands.
Conventional magnetic recording disks in hard disk drives typically use a continuous granular magnetic film, such as a sputter-deposited hexagonal-close-packed (HCP) cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetic bit in the medium is comprised of many small magnetized grains.
The challenge of producing continuous granular films as magnetic media will grow with the trend toward higher areal storage densities. Reducing the size of the magnetic bits while maintaining a satisfactory signal-to-noise ratio, for example, requires decreasing the size of the grains. Unfortunately, significantly reducing the size of weakly magnetically coupled magnetic grains will make their magnetization unstable at normal operating temperatures. To postpone the arrival of this fundamental xe2x80x9csuperparamagneticxe2x80x9d limit and to avert other difficulties associated with extending continuous granular media, there has been renewed interest in patterned magnetic media.
With patterned media, the continuous granular magnetic film that covers the disk substrate is replaced by an array of spatially separated discrete magnetic regions or islands, each of which serves as a single magnetic bit. The primary approach for producing patterned media has been to selectively deposit or remove magnetic material from a magnetic layer on the substrate so that magnetic regions are isolated from one another and surrounded by areas of nonmagnetic material. There are a variety of techniques for the selective deposition or removal of magnetic material from a substrate. In one technique the substrate is covered with a lithographically patterned resist material and a magnetic film is deposited to cover both the areas of resist and the areas of exposed substrate. The resist is dissolved to lift off the magnetic film that covers it, leaving an array of isolated magnetic regions. An alternative technique is to first deposit a magnetic film on the substrate and then pattern resist material on the magnetic film itself. Magnetic material from the areas not protected by the resist can then be selectively removed by well-known processes. Examples of patterned magnetic media made with these types of lithographic processes are described in U.S. Pat. Nos. 5,587,223; 5,768,075 and 5,820,769.
From a manufacturing perspective, an undesirable aspect of the process for patterning media that requires the deposition or removal of material is that it requires potentially disruptive processing with the magnetic media in place. Processes required for the effective removal of resists and for the reliable lift-off of fine metal features over large areas can damage the material left behind and therefore lower production yields. Also, these processes must leave a surface that is clean enough so that the magnetic read/write head supported on the air-bearing slider of the disk drive can fly over the disk surface at very low flying heights, typically below 30 nanometers (nm).
An ion-irradiation patterning technique that avoids the selective deposition or removal of magnetic material, but uses a special type of perpendicular magnetic recording media, is described by Chappert et al, xe2x80x9cPlanar patterned magnetic media obtained by ion irradiationxe2x80x9d, Science, Vol. 280, Jun. 19, 1998, pp. 1919-1922. In this technique Ptxe2x80x94Coxe2x80x94Pt multilayer sandwiches which exhibit perpendicular magnetocrystalline anisotropy are irradiated with ions through a lithographically patterned mask. The ions mix the Co and Pt atoms at the layer interfaces and substantially reduce the perpendicular magnetocrystalline anisotropy of the film, with the result that the regions of the disk that are not irradiated retain their perpendicular magnetic properties and serve as the magnetic bits.
Chemically-ordered alloys of FePt and CoPt formed as thin films have also been proposed for horizontal magnetic recording media. Chemically-ordered alloys of FePt and CoPt, in their bulk form, are known as tetragonal L10-ordered phase materials (also called CuAu materials). They are known for their high magnetocrystalline anisotropy and magnetic moment, properties that are also desirable for high-density magnetic recording media. The c-axis of the L10 phase is similar to the c-axis of HCP CoPt alloys in that both are the easy axis of magnetization. An ion-irradiated patterned disk that uses a continuous magnetic film of a chemically-ordered Co (or Fe) and Pt (or Pd) alloy with a tetragonal crystalline structure is described in IBM""s pending application Ser. No. 09/350,803 filed Jul. 9, 1999. The ions cause disordering in the film and produce regions in the film that are low coercivity or magnetically xe2x80x9csoftxe2x80x9d and have no magnetocrystalline anisotropy, so that the regions of the disk that are not irradiated retain their horizontal magnetic properties and serve as the magnetic bits.
One disadvantage of the Chappert et al. and IBM ion-irradiated patterned disks is that the regions separating the discrete magnetic regions from one another are not completely nonmagnetic, but still have some magnetic properties. Thus the magnetoresistive read head in the disk drive will detect noise and/or some type of signal from these regions. In addition, in these ion irradiation techniques the disk before ion-irradiation is ferromagnetic so that the regions of the disk that are not ion-irradiated become the regions that serve as the magnetic bits. This requires the use of a mask that is difficult to fabricate because the holes in the mask are used to generate corresponding nonmagnetic regions on the disk.
What is needed is a patterned magnetic recording disk that has discrete magnetic regions separated by completely nonmagnetic regions so that only the magnetic regions contribute to the read signal, and that is made by a patterning technique where the mask pattern of holes matches the pattern of discrete magnetic regions of the disk that serve as the magnetic bits.
The present invention is a magnetic recording disk wherein the magnetic recording layer is patterned into discrete magnetic and nonmagnetic regions having substantially the same chemical composition, but wherein the nonmagnetic regions have a chemically-ordered L12 crystalline structure and the magnetic regions having a chemically-disordered crystalline structure. The invention is based on the discovery that the chemically-ordered intermetallic compound FePt3, which is nonferromagnetic, can be rendered ferromagnetic by ion irradiation. With a clear transformation from nonferromagnetic to ferromagnetic, this FePt3 material can be patterned by irradiating local regions through a mask to create magnetic regions that serve as the magnetic bits. The ions pass through the openings in the mask and impact the chemically-ordered FePt3 in selected regions corresponding to the pattern of holes in the mask. The ions disrupt the ordering of the Fe and Pt atoms in the unit cell and transform the FePt3 into magnetic regions corresponding to the mask pattern, with the regions of the film not impacted by the ions retaining their chemically-ordered structure. The coercivity of the magnetic regions in the patterned media can be increased by slightly altering the 3:1 ratio of Pt to Fe by increasing the amount of Fe or by adding a third element, such as small amounts of cobalt (Co) and/or nickel (Ni), during the sputter deposition of the FePt3 film.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.